Microfibrous entrapped catalysts (MFECs) or microfibrous entrapped sorbents (MFESs) are known in the art and have been used for various applications. Several steps are required for preparing MFECs and MFESs including fiber mixing, conventional wetlay paper making, media sintering, and catalyst formation as described in U.S. Pat. Nos. 5,080,963, 5,096,663, 5,102,745, 5,304,330, 6,231,792 and U.S. Patent Application Publication No. 2011/0135543.
In the mixing step, the catalyst/sorbent support particles are mixed with micron-sized fibers of metal, polymer, glass, etc. in the presence of water or other liquids as needed. In order to achieve good mixing, the fibers are typically short; having lengths on the order of 2-3 mm. Viscosity modifiers can be added to improve mixing. In order to enhance the strength of the to-be-formed media, binders such as cellulose fibers are typically used. The mixture is then formed as green media on the screen of a sheet former that removes the liquid solution from the solids which typically include fibers, cellulose, and particles. The green media is then compressed to remove water and ensure the particles are well entrapped. Due to the presence of water and other contaminants in the water, some catalysts or catalysts supports are deactivated or poisoned.
After the green media is dried, it is ready to be sintered. In the sintering step, the micron-sized fibers are sintered together forming fiber-fiber junctions. The media form a sintered network locking entrapped particles inside. If the media is made of metal or glass fibers, the green media will be sintered at a high temperature. Hydrogen at a low dew point is typically used to ensure sufficient sintering. In this case, the cellulose fibers are converted to carbon fibers. If carbon fibers are not desired, a pre-oxidation step is typically needed. If the media is made of polymer fibers, it is typically sintered at 100-150° C. For this case a low-dew-point hydrogen atmosphere is not required, and cellulose fibers will remain inside the sintered media.
If pre-oxidation is required, the current practice is to expose the green media in an oxidative environment (typically oxygen lean air containing 5-10% O2 diluted by N2 or steam or other inert gas or combinations) at a temperature of 400-550° C. for 30-60 minutes. In the pre-oxidation step, the cellulose fibers are removed and fragile media is left behind. The media then goes through the sintering step and forms the sintered media.
Both the sintering step and pre-oxidation step require high temperatures and highly reductive or oxidative environments, respectively, particularly the sintering step. Most catalysts or catalyst supports are deactivated due to the loss of surface area and pore volume during sintering and/or pre-oxidation. Moreover, decomposition of cellulose fibers in the pre-oxidation step will generate hydrocarbon intermediates, which may poison catalysts.
In the catalyst formation step, catalyst precursors are loaded on the catalyst supported particles and are converted into active catalysts, typically with thermal treatments. This step varies greatly depending on the catalyst formulation. Corrosive gases such as NOx, HCl, and SOx are commonly present during catalyst formation, and these gases may damage the sintered media. Another major challenge associated with this step is that the detailed catalyst formation conditions must be known; however, this information is typically proprietary and not generally available to the public.
The prior art approaches require catalyst or catalyst support particles to be mixed with fibers in the fiber mixing step. After wetlay paper making, force is applied to compress the media and generate a dense media for particle immobilization. The dense media are then treated as described above. After the sintering step, if catalyst supports are used, they will be loaded with catalyst precursors followed by catalyst formation. During MFEC and MFES preparation, the catalysts or catalyst supports can be easily contaminated or deactivated, especially during fiber mixing in aqueous solutions and high temperature sintering, and the metal fiber structure may also contaminated by gases (e.g. NOx, SOx) generated during catalyst formation step. Moreover, the catalyst formation step requires the disclosure of catalyst formulation and preparation conditions, which are typically proprietary information and protected as trade secrets by most catalyst developers. These limitations significantly hinder the advancement of MFEC and MFES technology.
As discussed above, the process previously developed for MFEC preparation faces numerous technical difficulties. There exists a need to develop improved processes for the preparation of MFECs and MFESs.
Therefore, it is an object of the invention to provide improved processes for the preparation of MFECs and MFESs, particularly processes that overcome the limitations described above.